Epidemiological analysis of syringe sharing behaviors among IDUs are typically based on self-reports and are subject to socially desirable responding. We have developed and applied three short tandem repeat (STR) genetic biomarkers to detect sharing in 2512 syringes exchanged by 315 IDUs in the Baltimore needle exchange program (738 person-visits). Demographic characteristics, direct and indirect needle sharing behaviors corresponding to the closest AIDS link to the intravenous experience (ALIVE) study visits were examined for association with multi-person-use of syringes. Overall, 56% of the syringes exchanged at Baltimore needle exchange program (NEP) had evidence of multi-person-use of syringes. Less multi-person-use of syringes (48 vs. 71%, p<0.0001) was seen with more rapid syringe turnaround (<3 days). IDUs always exchanging their own syringes (primary syringes) were less likely to return syringes with evidence of multi-person-use (52%) than those who exchanged syringes for others (secondary syringes); (64%, p=.0001), and those exchanging both primary and secondary syringes (58%, p=.004). In a multivariate analysis restricted to primary exchangers, multi-person-use of syringes was associated with sharing cotton (AOR=2.06[1.30-3.28]), lending syringes (AOR=1.70[1.24-2.34]) and injecting less than daily (AOR=0.64[0.43-0.95]). These findings support additional public health interventions such as expanded syringe access to prevent HIV and other blood-borne infections. Testing of STRs represents a promising approach to examining and accessing complex behavioral data including syringe sharing. These results were published during the reporting period (Shrestha, S. et al., 2006). "Multiperson Use of Syringes Among Injection Drug Users in a Needle Exchange Program: A Gene-Based Molecular Epidemiologic Analysis." J Acquir Immune Defic Syndr 43: 335-343).